The present invention relates to a system for protecting a building against damage due to earthquake forces.
Earthquakes present a major public safety hazard to building occupants and persons on the streets below. Earthquakes also create a major economic liability for building owners and communities that depend on the continuity of building usage. Buildings must be protected against the effects of structurally damaging forces generated by the random ground movements of earthquakes. The building superstructure must be capable of responding to displacing forces due to earthquakes, yet remain stable during high wind conditions. In the inventor's previously granted patent, the contents of which are incorporated by reference, the maximum horizontal force tolerated by that earthquake protection system had a predetermined magnitude which was preestablished during design and which could not be changed once the structure had been constructed. By providing an adjustable maximum horizontal force, higher safety factors can be attained during low wind conditions that are most common.
The random motions generated by earthquakes sometimes result in forces which tend to force a twisting or torsional rotation of the superstructure about a vertical axis. Such torsional rotation causes undesirable additional relative displacement at the exterior columns and walls of the superstructure. The vertical component of earthquake forces also cause uplifting at some parts of the superstructure. These uplift forces introduce damaging impact forces once the uplift force has subsided and the lifted portions of the superstructure drop suddenly. These torsional and uplifting motions could also erode the safety factors that are utilized in calculating the design tolerances for the system.
Sequential earthquake jolts in a particular direction may result in a buildup of relative displacement of the building. Therefore, it would be desirable for an earthquake protection system to urge the building to move back towards its original position whenever possible after relative displacement has occurred. Such jolts can also cause instability in the columns of a building when the building's weight is held eccentric to the vertical axis of the columns. It is therefore desirable to maintain concentric loading in the columns.